In recent years, there has been a demand for interconnected wind turbine generators to contribute to a reduction of frequency fluctuations in a utility grid within a predetermined time (for example, within 30 seconds) from the occurrence of a disturbance in the utility grid (primary frequency response, hereinafter referred to as “PFR”).
To support the PFR, Patent Literature 1 describes further supplying the power output of wind turbine generators to a utility grid when the frequency of the utility grid has decreased.
More specifically, the following kinds of operation and control for wind turbine generators are available.
An example of a method for ensuring the power output of the wind turbine generators to be supplied to the utility grid at the occurrence of the disturbance described above is an operation that limits the power output of the wind turbine generators in advance in normal operation (deload operation). However, since the deload operation outputs limited electric power, the amount of power generated by the wind turbine generators decreases.
Another method is control (inertia control) that uses inertial energy (also referred to as inertia) stored in the rotors of the wind turbine generators as electric power to be supplied to the utility grid at the occurrence of the disturbance described above. However, since the inertial energy of the rotors is lost due to the inertia control, the inertia control sharply decreases the rotational speed of the rotors to cause, for example, unnecessary disconnection of the wind turbine generators, thus raising the possibility of decreasing the power generated.
The inertia control can reduce the limitation amount of power output in the deload operation (hereinafter referred to as “deloading amount”). On the other hand, the deload operation can reduce the amount of inertial energy to be lost due to the inertia control (hereinafter referred to as “the amount of inertia used”) by increasing the deloading amount.